How to strip-down a gun

Nick Stanning uses some springer strip-down skills and a few drop-in parts to breathe some new life into a clapped-out Crosman

Optimus Prime: The Crosman Optimus in .22

At a recent spring rifle event in the Midlands, I was handed a rifle by a friend – it was a Crosman Optimus in .22. “Try this and tell me what you think,” he said. I took the gun and loaded it, noticing the extremely high cocking effort. I shouldn’t have been completely surprised on firing it when it tried to remove my fillings.

The look of shock and fear must have been apparent on my face as he laughed, and asked if I could do anything to make it more user-friendly. I hurriedly put it in its case and said I’d see what I could do.

Wow, what a looker! The Optimus is now back in the prime of its shooting life after spending a few hours on the workbench

That was some while ago now, and it’s time to take this rifle apart and see what’s going on inside. This is a gun I haven’t worked on before and, given the cocking effort, I suspect there will be a lot of preload on the spring. Normally I would secure the gun in my lathe and use the tail-piece to take the spring pressure and dismantle the gun under full control, but as not everyone has a lathe at their disposal I thought I would use the homemade spring compressor that I showed you how to make on page 45 of the previous issue of Airgun Shooter.

Basic disassembly of the Optimus begins much like any other springer, with two forend screws and two action screws being removed

The Optimus is firmly in the budget end of the air rifle market, with potential owners looking for a fun plinker rather than their next HFT rig. From the box they perform adequately, but this one had been used a lot, and the deterioration in performance had been noted. It was now very tough to cock, and kicked like a mule with a hell of a clatter into the bargain!
The first thing to do is remove the action from the woodwork. This is a straightforward task with a screw on each side of the forend which holds the front, and two more holding the trigger guard and the rear of the action. Remove the action and place the stock and screws safely to one side.

A hollow-ground screwdriver is perfect for airgun use, allowing the entire screw to be filled and avoiding the risk of the tool slipping

Next we need to undo the barrel hinge pin so the barrel can be parted from the action. This benefits us, as it means we have less length to put in the spring compressor, and also frees the cocking link from the piston so that it can come out too.
Use a good quality screwdriver of the correct size. I use purpose-made gunsmith screwdrivers that have a parallel tip: this means the tip reaches the very bottom of the slot, which gives it more torque and vastly reduces the chances of slip. At the rear of the action is a plastic insert, there purely to tidy the appearance. To remove this, push the retaining pin through with a punch. It will pull free. The spring pressure is now being held by the bolt keeping the trigger group and rear block in place. Put the action into the spring compressor with the trigger facing up and towards the screw end. Wind the compressor in to take the pressure, then undo the retaining bolt.

Removing the end block grants access to the spring and piston, but be careful as you never know how much preload’s present

Once removed, unwind the compressor slowly and carefully until the rear block is out and the spring is free. The end block, trigger group, spring and piston can now all be removed from the cylinder. That’s it!
Now it’s time to take a look at what’s going on inside. The first job on any airgun strip is to remove all the factory grease and oil. I use engine degreaser available from any auto parts outlet. Liberal use of this with plenty of rags will soon have things in a state ready for closer inspection.

Nick used his homemade compressor to take up preload on the spring while removing the end block and trigger

The first thing I noticed was that there were some extreme signs of wear on the piston, with similar wear on the spring guide. This is almost certainly part of the problem with the excessive cocking effort. It’s due to poor lubrication, but is easily fixed. To remove the scratches and wear on the piston and guide, I polished them, starting with medium-grade wet and dry paper, then fine-grade, and finished off with some metal-polishing compound.

When reassembling any rifle, lay out the parts and be sure to get their orientation right before trying to fit everything together

Next I had a look at the top hat: this weighs a whopping 33g and accounts for 15% of the piston mass! Checking the spring, I found that it is the same internal diameter as the one used in the Walther Terrus. I have some TbT top hat and washer kits for those, so I replaced the heavy steel for a plastic part weighing just 6g, which has reduced the moving mass by 27g. This will have a very positive effect on the recoil.

Nick sorted out the friction problems and polished the damaged parts before applying some good quality moly-based lube

The kit also includes a slip washer that goes in the piston ahead of the top hat and spring. This acts as a bearing, allowing the spring to twist in a natural manner as it expands. The other washers can be used to increase the preload if required.
Now it’s time to rebuild the rifle. To give lasting lubrication, I used a high-moly-content paste in very small amounts on the outside of the piston, on the spring and on the rear guide. It really is a case of less in more in this respect. Too much, and the lube will get ahead of the piston seal, causing an effect known as dieseling. The pressure in the compression tube reaches such high levels that any combustible material in there can ignite, like in a diesel engine. This leads to inconsistent performance and seal wear, and can damage the major components.

The piston and spring guide exhibit extreme signs of wear, largely due to inadequate lubrication – a problem that’s easily fixed

Before using the spring compressor to refit the rear block, the trigger group needs to be in the correct place. When lining it up, be sure to have the sear resting on the outside of the cylinder. The rest is a simple matter of reversing the strip-down sequence.
Previously, the gun had taken lots of effort to cock and recoiled to such a degree that it made the shooting experience quite unpleasant. Now that the internal friction issues have been fixed, the cocking effort is noticeably reduced; and the weight loss in the piston, along with the reduced torsional recoil, has improved the firing cycle to a more-than-acceptable level.

Tools of the trade: what you need for a strip-down

1. A purpose-made gunsmith’s toolkit isn’t cheap, but it’s well worth the investment if you aim to look after your guns and equipment yourself.

2. Gunsmith’s screwdrivers have hollow-ground parallel tips to ensure a good fit into the screw, for maximum torque without slipping.

3. A brass hammer and punches allow you to work safely on your rifle without the risk of marking the blued steel finish.

4. This is a top hat – a small spring guide that supports the spring within the piston.

5. A good-fitting rear guide supports the rear of the spring and allows a controlled expansion for
twang-free firing.

6. Various grades of moly grease are readily available. Use as little as possible: the higher the moly content, the better the lubrication.

7. A chronograph measures the power of your airgun, and is an essential piece of equipment for anyone wishing to work on their guns.


This article originally appeared in the issue 99 of Airgun Shooter magazine. For more great content like this, subscribe today at our secure online store www.myfavouritemagazines.co.uk

Tagged with: , , , , , , , , ,
Posted in Features, How to

Leave a Reply

Your email address will not be published.

*

Follow Us!